A surface protective sheet is bonded to a metal plate, a decorative plastic plate, a glass plate or the like through an adhesive layer in order to prevent the formation of scratches or fractures on the surface thereof during storage or transfer, or to protect the metal surface during working. Such surface protective sheets are described in detail, for example, in U.S. Pat. No. 3,600,268.
The surface protective sheet is rolled or wound on a reel and supplied in that state as a pressure-sensitive tape. That is, an adhesive layer on a sheet substrate is in contact with the reverse side of the sheet substrate. It is necessary therefore to provide a release coating layer on the reverse side of the sheet substrate so that the adhesive layer and the reverse side of the sheet substrate in contact therewith can be easily separated from each other. The release coating layer is further required to be firmly bonded to the reverse side of the sheet substrate so as to be not peeled off by the adhesive layer, because if the release coating layer is peeled off by the adhesive layer, when the surface protective sheet is bonded to a metal plate or the like, the release coating layer peeled off attaches to and stains the surface of the metal plate or the like.
Compositions which have heretofore been used to provide such a release coating layer on a substrate for surface protective sheets and pressure-sensitive adhesive tapes include a solution of an octadecyl (meth)acrylate-(meth)acrylic acid copolymer, silicone solutions, etc.
Release coating layers produced from these known compositions are not completely satisfactory; for example, release coating layers produced from the copolymer solutions are inferior in their release characteristics (i.e., in preventing the adhesive layer from sticking to the reverse side of the substrate), and although the silicone solutions provide release coating layers having excellent release characteristics, the excellent release characteristics contrarily deteriorate other characteristics.
When the silicone solution is coated on the sheet substrate and baked, the adhesion between the release coating layer formed and the sheet substrate is not sufficient because of its inherent physical properties. The silicone release coating layer formed, therefore, easily peels off. In particular, when the silicone solution is applied onto a plasticized polyvinyl chloride sheet substrate, the peeling-off of the silicone release coating layer is accelerated by the transfer of plasticizers thereinto from the sheet substrate. This gives rise to the problems that the release characteristics of the release coating layer is reduced, and that when the surface protective sheet obtained is bonded to a metal plate or the like, fine flakes of the coating film peeled off attaches to and stains the metal plate or the like.
Furthermore, the foregoing compositions are generally used as a solution using an organic solvent. The use of such organic solvents, however, is not desirable in that they may cause fire, explosion, etc., and, furthermore, they are somewhat toxic and may cause air pollution.
An aqueous composition has been proposed to provide a release coating layer without the use of an organic solvent, as described in Japanese Patent Application (OPI) No. 139835/76 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). This aqueous composition is to be applied onto, in particular, a permeable substrate, such as paper, non-woven fabric, and cloth. That is, by application of the aqueous composition, back-sizing due to partial permeation of the aqueous composition into the permeable substrate, and formation of a release coating layer on the substrate can be attained at the same time. The aqueous composition comprises 100 parts by weight, based on solids content, of a resin emulsion and 0.5 to 100 parts by weight, based on solids content, of a silicone emulsion. It has been found, however, that when the aqueous composition is applied onto a non-permeable sheet substrate, a complete silicone coating layer cannot be formed as an uppermost layer, resulting in the formation of a release coating layer having poor release characteristics. Furthermore, the release characteristics of the release coating layer are markedly reduced when the release coating layer is allowed to stand at high temperatures and humidities for long periods of time.